Searching the internet and this forum I found some information about the housing of the rear fuel filter but nothing about the filter element itself. Can anyone advise about supplier and parts number? Is it a Fispa cartridge and are these still available?

Yes it is indeed a Fispa cartridge as I still have the original one out of my 250 ... the one that had NEVER been changed! It's equivalent is a Fram C4518 or a UFI 26-617-00 ... both of which come with new springs as part of the cartridge that keeps them seated in the proper position in the canister.

I know Mr. Fiat sells the UFI filter (under their own number of course which I believe to be FR-208-322 under one of their Fiat listings but you would have to confirm that with David) but I'm not sure who is supplying them.